1. Field of the Invention
The present invention concerns a circuit for connection of at least two local coils of a magnetic resonance apparatus with at least two evaluation units via two separate lines (conductors).
2. Description of the Prior Art
In magnetic resonance examinations, magnetic resonance signals of an examined patient frequently are acquired by means of local coils positioned close to the patient. The radio-frequency signals are acquired by the local coils and transmitted via a circuit arrangement to the evaluation units of the magnetic resonance apparatus. Arrangements known as local coil arrays are composed of multiple local coils that are simultaneously operated, which leads to interferences due to RF coupling between signal lines in the circuit arrangement. The signal lines generally are shielded to prevent such interferences. An example of a shielded signal line is a coaxial cable. Given the use of such cables, however, the problem exists that sheath waves and sheath wave currents that cause interferences can propagate on the shield conductor. Large sheath wave currents can lead, for example, to an endangerment of the patient or interfere with components. One possibility to prevent such interferences is the use of sheath wave barriers. These can be formed, for example, by windings of the shielded cable formed into a coil and connected in parallel with a capacitive element. The parallel circuit represents a high impedance for sheath waves so that propagation thereof is prevented. It is too complicated, however, to provide every signal line with an individual shield and sheath wave barrier. In particular the cramped space relationships in the imaging apparatus represents a limitation as to the use of sheath wave barriers.
One possibility is to provide a number of cables with a common shield and to decouple them via one sheath wave barrier. Interferences still result, however, due to the cables being wound into coils, for example on further cables or local coils.